The present invention relates, in general, to the field of preamplifier circuit design. More particularly, the present invention relates to a low noise preamplifier for a magnetoresistive ("MR") data transducer, or "read" head which reduces the noise contributions of the load resistor in a first stage of the preamplifier in addition to lessening that noise otherwise contributed by the second gain stage of the preamplifier.
Magnetoresistive read heads are known to be useful in reading data encoded on a magnetic surface with a sensitivity exceeding that of inductive or other thin-film heads. In operation, an MR head is used to detect magnetic field signal changes from a magnetic surface due to the fact that the resistance of the MR sensor changes as a function of the direction and amount of flux being sensed. It is also generally known that for an MR transducer to function effectively, it must be subjected to a transverse bias field to linearize its response, in addition to a longitudinal bias to minimize Barkhausen noise. Various techniques for effectuating such transverse biasing are known, including current shunt biasing and soft adjacent film biasing. The transverse bias field is applied normal to the plane of the magnetic media and parallel to the surface of the MR sensor.
In current MR read head preamplifier design, the current through the head is fixed and a function of the resistance of the read head itself. As is known, a critical aspect of any preamplifier design is its noise figure, which, in the case of a conventional preamplifier design, is a function of the value of the load resistor (R.sub.L). To a first approximation, the output noise due to RL is proportional to 1/sqrt (R.sub.L). Because in conventional circuit designs the total current through the MR read head is applied through R.sub.L, it is not possible to merely increase the value of R.sub.L to reduce the output noise of the preamplifier without concomitantly increasing the voltage drop across R.sub.L to an undesirable level.